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Popa, Constantin 한국전산응용수학회 1999 Journal of applied mathematics & informatics Vol.6 No.1
We give a new characterization of the solutions set of the general (inconsistent) linear least-squares problem using the set of linit-points of an extended version of the classical Daczmarz's pro-jections method. We also obtain a "step error reduction formula" which in some cases can give us apriori information about the con-vergence properties of the algorithm. Some numerical experiments with our algorithm and comparisons between it and others existent in the literature are made in the last section of the paper.
A FAST KACZMARZ-KOVARIK ALGORITHM FOR CONSISTENT LEAST-SQUARES PROBLEMS
Popa, Constantin 한국전산응용수학회 2001 The Korean journal of computational & applied math Vol.8 No.1
In some previous papers the author extended two algorithms proposed by Z. Kovarik for approximate orthogonalization of a finite set of linearly independent vectors from a Hibert space, to the case when the vectors are rows (not necessary linearly independent) of an arbitrary rectangular matrix. In this paper we describe combinations between these two methods and the classical Kaczmarz’s iteration. We prove that, in the case of a consistent least-squares problem, the new algorithms so obtained converge ti any of its solutions (depending on the initial approximation). The numerical experiments described in the last section of the paper on a problem obtained after the discretization of a first kind integral equation ilustrate the fast convergence of the new algorithms. AMS Mathematics Subject Classification : 65F10, 65F20.
AVERAGES AND COMPACT, ABSOLUTELY SUMMING AND NUCLEAR OPERATORS ON C (Ω)
Popa, Dumitru Korean Mathematical Society 2010 대한수학회지 Vol.47 No.5
In the paper we introduce averages of each type and use these averages to construct examples of weakly compact operators on the space C ($\Omega$) for which the necessary and sufficient conditions that they be compact, absolutely summing or nuclear are distinct. A great number of concrete examples, in various situations, are given.
PRECONDITIONED KACZMARZ-EXTENDED ALGORITHM WITH RELAXATION PARAMETERS
Popa, Constantin 한국전산응용수학회 1999 Journal of applied mathematics & informatics Vol.6 No.3
We analyse in this paper the possibility of using preconditioning techniques as for square non-singular systems, also in the case of inconsistent least-squares problems. We find conditions in which the minimal norm solution of the preconditioned least-wquares problem equals that of the original prblem. We also find conditions such that thd Kaczmarz-Extendid algorithm with relaxation parameters (analysed by the author in [4]), cna be adapted to the preconditioned least-squares problem. In the last section of the paper we present numerical experiments, with two variants of preconditioning, applied to an inconsistent linear least-squares model probelm.
A GENERAL COMMON FIXED POINT THEOREM FOR TWO PAIRS OF MAPPINGS IN METRIC SPACES
Popa, Valeriu,Patriciu, Alina-Mihaela The Honam Mathematical Society 2018 호남수학학술지 Vol.40 No.1
In this paper a general fixed point theorem for two pairs of mappings involving altering distance is proved. This theorem generalizes Theorem 9 [5], Theorems 1, 2, 3 [6], Theorems 2.3, 2.4 [7] and other results from [11]. As applications, some results for mappings satisfying contractive conditions of integral type and ${\phi}$-contractive conditions are obtained.
A GENERAL COMMON FIXED POINT THEOREM FOR TWO PAIRS OF MAPPINGS IN METRIC SPACES
( Valeriu Popa ),( Alina-mihaela Patriciu ) 호남수학회 2018 호남수학학술지 Vol.40 No.1
In this paper a general fixed point theorem for two pairs of mappings involving altering distance is proved. This theorem generalizes Theorem 9 [5], Theorems 1, 2, 3 [6], Theorems 2.3, 2.4 [7] and other results from [11]. As applications, some results for mappings satisfying contractive conditions of integral type and Ø - contractive conditions are obtained.
AVERAGES AND COMPACT, ABSOLUTELY SUMMING AND NUCLEAR OPERATORS ON C (Ω)
Dumitru Popa 대한수학회 2010 대한수학회지 Vol.47 No.5
In the paper we introduce averages of each type and use these averages to construct examples of weakly compact operators on the space C (Ω) for which the necessary and sufficient conditions that they be compact, absolutely summing or nuclear are distinct. A great number of concrete examples, in various situations, are given.
Monica Popa,Ecaterina Vasilescu,Paula Drob,Doina Raducanu,Jose Maria Calderon Moreno,Steliana Ivanescu,Cora Vasilescu,Silviu Iulian Drob 대한금속·재료학회 2012 METALS AND MATERIALS International Vol.18 No.4
For an alloy to be suitable for use as an implant material, it must have a low specific weight and Young’s modulus, good mechanical properties that are similar to those of bone, and very good corrosion resistance and biocompatibility. In this study, we have developed a novel Ti-20Nb-10Zr-5Ta alloy that is composed of nontoxic, nonallergenic, corrosion-resistant elements. This alloy has low specific weight and Young’s modulus and good mechanical properties. It has a fine microstructure with a matrix that is mainly composed of the β phase and some α phase due to recrystallization during cooling. It shows elastoplastic behavior with a fairly linear elastic behavior and low Young’s modulus (59 GPa). In addition, its ultimate tensile strength, 0.2% yield strength, and hardness are higher than those of CP Ti, commercial Ti-6Al-4V,and similar β-type alloys. It exhibited a very stable passive state and its electrochemical parameters and corrosion and ion release rates were better than those of CP Ti in Ringer’s solutions of different pH values that simulate the severe functional conditions of an implant; this is attributable to the beneficial influence of the alloying elements and to the better protective properties of the coated passive film.
Daniel Popa,Jayapal Ramesh,Shajan Peter,C. Mel Wilcox,Klaus Mönkemüller 대한소화기내시경학회 2014 Clinical Endoscopy Vol.47 No.1
Self-expanding metal stents are a useful therapy to palliate malignant and benign luminal gastrointestinal obstruction. Self-expanding metal stents has been widely reported for colonic, esophageal, and gastric obstruction. However, endoscopic delivery and placement to the small bowel is more challenging and difficult. This case illustrates the usefulness and technical advantages of the balloon-overtube and enteroscopy technique for the palliative treatment of neoplastic stenosis affecting the small intestine.